1. Field of the Invention
The present invention generally relates to a cable connector, and more particularly, a cable connector for a balanced transmission having a relay wiring substrate in which a contact assembly is joined to the relay wiring substrate, and an end of a cable is connected to an end of the relay wiring substrate.
2. Description of the Related Art
As for methods of data transmission, for example, a normal transmission method is provided in which one wiring is used for each data signal. Further, a method of a balanced transmission is provided in which a pair of wirings are used for each data signal. Through the pair of wirings, a plus signal and a minus signal are simultaneously transmitted, which minus signal has the same size as the plus signal and a polarity reverse to the plus signal. The method of the balanced transmission has an advantage in that the balanced transmission is less susceptible to noise than the normal transmission method. Accordingly, the method of the balanced transmission has been adopted in fields where signals are transmitted at high speed.
As an industry standard, the impedance of a transmission path of the signal is a predetermined value, for example, 100 Ω. The transmission path of the signal includes the cable connector portion for balanced transmission in addition to the cable portion for balanced transmission. It is ideal that the impedance be equal to 100 Ω over the total length of the transmission path of the signal. Especially, in the case where the signal is transmitted at a high speed, a change of the impedance at even one part may adversely affect the signal.
Accordingly, as for the cable connector for balanced transmission, it is desirable that the change of the impedance at each part be as small as possible.
FIG. 1 is a view schematically illustrating a conventional cable connector 10 for balanced transmission disclosed in Japanese Laid-Open Patent Application Publication No. 2003-059593. FIG. 2 is a cross-sectional view illustrating a relay wiring substrate 20 and an enlarged part where the relay wiring substrate 20, the cable 40 for balanced transmission and a contact assembly 11 are connected together. FIGS. 3A and 3B are perspective views illustrating the relay wiring substrate 20. The lines X1-X2, Y1-Y2, and Z1-Z2 show a width direction, a length direction, and a height direction of the cable connector 10 for balanced transmission, respectively. Y1 shows a back direction, while Y2 shows a front direction thereof.
As for the cable connector 10 for balanced transmission, the relay wiring substrate (paddle card) 20 is connected to the back of the contact assembly 11. An end of the cable 40 of balanced transmission is connected to an end of the relay wiring substrate 20. The contact assembly 11, the relay wiring substrate 20, and a tip end of the cable 40 are covered by a shield housing 50.
The relay wiring substrate 20 has a two-layer structure sandwiching a ground layer 22 over an entire area between the two layers. As shown in FIG. 3A, the relay wiring substrate 20 further comprises, on an upper surface 21a thereof, a first contact connecting pad 23, a first wiring connecting pad 24, and a first wiring pattern 25 connecting the first contact connecting pad 23 and the first wiring connecting pad 24. As shown in FIG. 3B, the relay wiring substrate 20 further comprises, on an under surface 21b thereof, a second contact connecting pad 33, a second wiring connecting pad 34, a second wiring pattern 35 connecting the second contact connecting pad 33 and the second wiring connecting pad 34, and a second ground pattern 36. As shown in FIG. 2, a distance “A” between the ground layer 22 and each of the first and second contact connecting pads 23 and 33 is equal to a distance “B” between the ground layer 22 and each of the first and second wiring connecting pads 24 and 34. Returning to FIGS. 3A and 3B, first and second ground patterns 26 and 36 both have a comb shape. The first ground pattern 26 comprises first partitioning patterns 27 respectively extending between the first wiring patterns 25, and a first Y1 side connecting pattern 28 on the Y1 side extending in the X1-X2 direction for connecting an end of the respective first partitioning patterns 27 on the Y1 side. In the same manner, the second ground pattern 36 comprises second partitioning patterns 37 respectively extending between the second wiring patterns 35, and a second Y1 side connecting pattern 38 on the Y1 side extending in the X1-X2 direction for connecting an end of the second partitioning patterns 37 on the Y1 side.
As shown in FIG. 1, the cable connector 10 for the balanced transmission includes a section 60 where the first and second wiring patterns 25 and 35 are provided, a section 70 where the first and second wirings 41 and 42 are respectively soldered onto the first and second wiring connecting pads 24 and 34, and a section 80 where the first and second signal contacts 13 and 14 are respectively soldered onto the first and second contact connecting pads 23 and 33.
As for the section 60, the desired value of the impedance Ip60 is equal to 100 Ω.
As for the section 80, however, it is found that the impedance Ip80 is slightly greater than the desired value, i.e., 100 Ω in relation to a shape and a volume of the first and second finger portions 13b and 14b of the first and second signal contacts 13 and 14, which portions 13b and 14b are respectively soldered onto the first and second contact connecting pads 23 and 33.
In addition to this, as for the section 70, as shown by a circle “A” in the enlarged view of FIG. 2, it is found that corresponding parts of the first and second wirings 41 and 42 face the first and second Yl side connecting patterns 28 and 38 so that the impedance Ip70 is slightly different from the desired value, i.e., 100 Ω. Namely, the first and second Y1 side connecting patterns 28 and 38 have an influence on the impedance Ip70.